Interannual Variability and Trends of Extratropical Ozone. Part I: Northern Hemisphere

Abstract

The authors apply principal component analysis (PCA) to the extratropical total column ozone from the combined merged ozone data product and the European Centre for Medium-Range Weather Forecasts assimilated ozone from January 1979 to August 2002. The interannual variability (IAV) of extratropical O3 in the Northern Hemisphere (NH) is characterized by four main modes. Attributable to dominant dynamical effects, these four modes account for nearly 60% of the total ozone variance in the NH. The patterns of variability are distinctly different from those derived for total O3 in the tropics. To relate the derived patterns of O3 to atmospheric dynamics, similar decompositions are performed for the 30?100-hPa geopotential thickness. The results reveal intimate connections between the IAV of total ozone and the atmospheric circulation. The first two leading modes are nearly zonally symmetric and represent the connections to the annular modes and the quasi-biennial oscillation. The other two modes exhibit in-quadrature, wavenumber-1 structures that, when combined, describe the displacement of the polar vortices in response to planetary waves. In the NH, the extrema of these combined modes have preferred locations that suggest fixed topographical and land?sea thermal forcing of the involved planetary waves. Similar spatial patterns and trends in extratropical column ozone are simulated by the Goddard Earth Observation System chemistry?climate model (GEOS-CCM). The decreasing O3 trend is captured in the first mode. The largest trend occurs at the North Pole, with values ??1 Dobson Unit (DU) yr?1. There is almost no trend in tropical O3. The trends derived from PCA are confirmed using a completely independent method, empirical mode decomposition, for zonally averaged O3 data. The O3 trend is also captured by mode 1 in the GEOS-CCM, but the decrease is substantially larger than that in the real atmosphere.